The Effect of Heat and pH
on Vegetables!
Eileen Huang/8 Mrs. Gillum/Marshall Middle School Introduction, Statement of Problem, and Purpose
It is necessary to process vegetables to preserve them as a year-round food source. There are many types of
ways to conserve them but the most common kind is to use thermal-processing or to just simply can them. To do this, the
vegetables are trimmed, washed, put into cans, and sealed for heating. It is necessary to heat it because it destroys to
microorganisms that cause spoilage and disease. However, this heating process can cause textural and chemical change to
the vegetable. The textural change is cause by damage in the cell’s cell wall and cell membrane and the chemical change
happens when the chlorophyll is altered resulting in a less pigment. This experiment shows how vegetables react to
different types of heat such as frying, boiling, steaming, etc.
Green vegetables contain the pigment called chlorophyll. Other vegetables contain the pigments
carotenoids, chlorophylls, and flavonoids. Chlorophyll plays a key role in transferring light energy to chemical energy
during the development and growth of the plant by the process of photosynthesis. Such vegetables include spinach,
beans, peas, cabbage, lettuce, and celery. When heat is added to such vegetables, many reactions occur. For example,
texture and pigment change. Many experiments have been done on testing which vegetable has which reaction with heat.
A common final product is having the pigment either darken or lighten and have the texture rougher or smoother. These
outcomes damages the vegetables, texture and pigment-wise.
A common problem is damage to the cell structure. Since vegetables are preserved as a year-round food
source, they are many methods of preserving vegetables. Common techniques include thermal processing and canning
them. To thermal process, however, heat is applied to the organism which is what damaged the vegetable’s cell structure.
Without applying heat, microorganisms known to cause disease and spoilage are present. Also, once heat is applied to
these vegetables, texture and color change is often present. Therefore, people will not think the vegetables are not
aesthetically pleasing or fresh.
The purpose of this experiment is to see how the different ways of cooking vegetables can have different
effects on the color and texture of the vegetable. The experiment will focus on four different heat processes and two pH
related procedures. The hope from by doing this experiment is that one will find a better way of cooking vegetables so
that it won’t deflate a consumer’s appetite. A better way to cook the vegetables is a way that does not change the
vegetable’s color and texture drastically and a way that does not make the vegetables look not fresh and unappealing.
Previous Experiments
An experiment was conducted on the effect of heat and pH on vegetable pigment. By adding ingredients to purred
vegetables, which have very extreme and significantly different levels of pH, the ability to tell if pH has any result on vegetable
color at all will be open. The vegetable pH varied after each experiment. For example, the broccoli had a pH of 5.0 after puree, a
pH of 5.3 after steaming, a pH of 4.8 after roasting, a pH of 5.3 after frying, and a pH of 4.9 after boiling.
This led to them conclude that heat does seems to change the pigments in a couple of ways. The process of heating
does not change the structure of membrane proteins, but a way that the shade changed was by increasing the motions of the protein
atoms that circulate. The pH would change because the hydrogen gradient in the vegetable proteins would change. The gradient
would differ because the hydrogen ions, previously from the hydrogen bonds would move across the protein membrane.
Food scientists that did the process of the experiment observed that the vegetables had a chemical reaction with the
chemicals being used on them. For example, the pH varied and the texture and pigment also changed. They also noticed in their
first experiment that the example of peas with NaOH and distilled water has the highest average pH that is 12.463, followed by
peas with NaOH and cold distilled water that is 12.458, which are very close. When the scientists were analyzing the color changes
of the vegetables, they realized that half of the vegetables had similar color ranges while the other half also had similar color
ranges.
Besides the testing of green vegetables, the food scientists of this experiment also tested colored vegetables. The
results after each procedure were almost identical. The pH levels of beets, carrots, and broccoli were all less than an initial unit of 1.
For example, after the beets, carrots, and broccoli were steamed and pH tested, the pH levels were 5.3 for broccoli, 6.0 for beets,
and 5.6 for carrots. Regardless of how heat was applied to the vegetables, the pH levels did not vary too much after each procedure.
In conclusion, pH changed between green vegetables and colored vegetables aren’t too far apart.
The food scientist Gao-Feng Yuan observed that all results from the heat showed a significant loss of chlorophyll
and vitamin C in the vegetables when conducting this experiment. Frozen samples of broccoli and fresh broccoli were both
observed and differences were seen. In conclusion, The chlorophyll content in boiled, stir-fried, and microwaved broccoli was
reduced by 27%, 23%, and 18%. However, it was technically unchanged in the steamed broccoli.
This led to the understanding of the different reactions that occurred when broccoli was boiled, stir-fried,
microwaved, and steamed. Heat can change a vegetable’s pigment by intensifying and decreasing its color. Many examples were
showed in this experiment. For example, the experiment explained that heat can change the vegetables in many ways like boiling,
frying, and baking. These procedures can lead to the plant’s pigment can change along with its texture.
Core Science
Many plant pigments, especially chlorophyll and the anthocyanins or a flavonoid, are sensitive to heat and changes
in pH. Under favorable acid conditions, these pigments will show an acceptable color. However when the pH is increased or
decreased, the pigment may change to an undesirable color. This presents a sensory defect in the food. The objective of this
exercise is to determine the effect of heat and pH on plant pigments.
The basic structural unit of all plant tissues is the cell. Surrounding these cells is the cell walls that provide an elastic
support for the contents of the cell. The cell also contains a cell membrane which is located in the cell wall and controls the passage
of liquids into and out of the cell. The jelly-like substance that fills the cell is called the cytoplasm, which is made up of protein,
sugars, salts, and other substances dispersed into water. Located within the cytoplasm are also the separate inclusion bodies, named
plastids, which contain chlorophyll.
Green vegetables contain the green pigment also known as chlorophyll, which plays a key role in the process called
photosynthesis. That is transferring light energy to chemical energy during the growth and development of the plant by the process
of photosynthesis. Photosynthesis provides the basic energy source for virtually all organisms. An extremely important byproduct
of photosynthesis is oxygen, on which most organisms depend. Apart from the chlorophyll in green vegetables, colored vegetables
also play a big role in the effects of heat damage to vegetables. The four most common pigments include betalins, carotenoids,
flavonoids, and chlorophyll. Betalins are common in fungi and flowers and includes the hues ranging from red-violet and orange to
yellow. Betalins are established food colorants and are the cause of the deep color in beets.
Carotenoids or carotene is a pigment that absorbs blue and indigo light and provides rich yellows and oranges. Flavonoids
are located in the cytoplasm and plastids of flowers and fruit. These chemicals lower cholesterol levels and many have antioxidant
properties, so this pigment is also very important to our diet. Flavonoids were referred to as Vitamin P from the mid-1930 to the
early 50s but the term has fallen out of use.
High temperatures can cause numerous chemical and textural changes in the vegetables. These textural changes are
due to partial destruction of the cell wall and the cell membrane. The heat treatments can cause chemical alteration of the
chlorophyll, which results in a processed vegetable with less green color. Green vegetables turn brown during cooking due to
chlorophyll degradation into the brown phaeophytin. Numerous treatments can’t prevent chlorophyll degradation into other
compounds. Some treatments like alkaline cooking water are accompanied by loss of nutritional value and change of texture.
Cooking vegetables is a treatment that changes texture from hard to soft. Texture change is due to change in the
organization of the plant fibers, among other reasons. During the heat treatment, chlorophyll changes into a brown degradation
product i.e. chlorophyll changes into phaeophytin after the loss of magnesium in the center of the chlorophyll molecule. The
appearance of the brown color too shows the when a vegetables is overcooked, it has lost some of its nutritional values, since
cooking degrades cell walls and cell membranes as well.
Hypothesis
Previous experiment’s conclusions have showed that heat damages vegetables tremendously. Many plant pigments
such as carotenoids, chlorophylls, and flavonoids are sensitive to heat which causes damage in the cell. This means that the look
and feel of the vegetables will change. An assumption is that the color and texture change will affect the consumer’s appetite. This
experiment will test which heat process will have the most impact on the consumer’s appetite.
The hypothesis of this project is that the pH levels of the vegetables will raise less than an initial unit of 1.0 pH
for all vegetables. This meaning that each vegetable will either raise or lower an initial unit of 1.0 pH. After being steamed, the
vegetables are predicted to raise only 0.2 units. Once being boiled, the vegetables are predicted to raise 1.0 unit. Roasted, the
vegetables are predicted to lower 0.2 units. When fried, the vegetables are predicted to raise 0.3 units. After being boiled, the
vegetables are predicted to lower 0.1 units.
Materials
The materials used during this experiment included petri dishes, 32 vegetables, baking soda, acetic acid, a pH meter,
and a camera. At first, three vegetables were tested per trial. These three vegetables were sliced, blended, and baking soda and
acetic acid was added. After the pH testing was finished, the vegetable slices that hadn’t been blended yet were heated with the
processes including roasting, frying, boiling, and steaming. These vegetables that had heat applied to it was late also pureed and
their pH levels were found.
Procedures
1) Rinse the vegetables and cut it into slices.
2) Put the slices into seven separate containers. Be sure to evenly distribute the vegetable slices among the containers.
3) Blend the first container of the vegetables into puree and take a picture for reference.
4) Blend the second container into a puree and add acetic acid then wait 10 minutes and take a picture.
5) Blend the third container into a puree and add baking soda then wait 10 minutes and take a picture.
6) Take the fourth container and fry the vegetables for two minutes then blend it into a puree
Procedures, cont.
7) Take the fifth container and roast the vegetables for two minutes then blend it into a puree.
8) Take the sixth container and steam the vegetables for two minutes then blend it into a puree.
9) Take the last container and boil the vegetables for two minutes then blend it into a puree.
10) Repeat the steps above for the other vegetables.
11) Using a pipette, maintain a small amount and put one drop onto the tip of the pH meter.
12) Repeat the step above for all of the vegetable purees including acetic acid, baking soda, roasted, fried, steamed, and boiled.
13) Record the pH levels.
Observations and Results
Vegetable Control Baking Soda Acetic Acid Roasted Boiled Steamed Fried Artichoke Asparagus Beets Bitter Melon Broccoli Brussel Sprouts Cabbage Carrots Cauliflower Celery Chives Corn Cucumber 6.09 5.60 6.20 6.04 6.58 6.42 6.62 6.61 7.04 5.86 6.8 5.2 6.3 7.69 7.65 7.67 7.52 8.12 8.33 7.93 8.16 8.20 7.76 8.05 7.33 7.14 4.34 4.20 4.83 4.8 4.43 4.62 4.56 4.86 4.7 4.44 5.2 4.66 4.94 5.82 5.98 6.39 5.85 6.53 6.84 5.81 6.13 6.22 5.86 5.85 5.16 6.52 5.80 6.41 6.12 5.94 6.71 6.03 6.54 6.4 6.64 5.87 5.93 5.32 6.73 6.53 6.14 6.22 6.16 6.9 6.49 6.3 6.29 6.73 6.33 5.85 5.30 6.6 5.67 5.85 6.3 5.94 6.5 6.08 6.69 6.56 5.39 5.91 5.72 5.86 6.09 Observations and Results, cont.
Cucumber Eggplant Green beans Green bell pepper Kale Leek
Mushroom Onion Parsley Peas
Pumpkin Radish Red bell pepper Spinach Sweet potato Tomato Turnip Yam Yellow bell pepper Zucchini 6.3 5.62 4.95 5.84 5.82 6.48 6.33 5.32 6.18 6.87 6.90 6.60 5.13 5.28 5.93 4.68 7.77 6.55 5.12 6.77 7.14 8.06 6.51 7.32 8.17 7.96 7.57 7.06 8.28 7.70 7.74 7.92 7.17 8.17 7.24 7.56 7.48 7.99 6.89 7.59 4.94 4.48 5.10 5.18 4.93 4.51 4.96 4.59 4.22 5.46 4.46 4.61 4.67 4.93 5.02 4.33 4.74 4.68 4.27 5.08 6.52 5.62 4.98 5.56 6.17 5.91 6.34 5.19 6.38 7.07 6.31 5.79 5.03 6.57 5.84 4.75 5.78 5.91 5.15 6.25 6.73 6.93 5.02 6.0 6.67 6.32 6.83 6.32 7.53 6.98 6.22 6.24 5.16 6.8 5.83 4.81 5.96 5.86 5.10 6.94 6.6 5.98 5.33 6.34 6.89 6.42 6.61 5.36 6.63 6.91 6.70 7.10 5.26 7.0 6.21 4.59 6.31 5.78 5.10 6.79 6.09 5.96 5.37 5.58 6.26 6.19 6.66 5.53 6.86 6.94 5.88 6.53 4.91 6.65 5.69 4.86 5.88 5.79 5.14 6.1 Observations and results, cont.
Not many of the vegetables had a very noticeable color change after to various tests like the
carrots, but some had a very noticeable color change. The carrot’s orange pigment had lightened just
barely after acetic acid was added, still counting as a color change, but not as much as some of the other
vegetables. For example, after the radishes were pureed and baking soda or acetic acid was added, the
pigment of the puree changed very prominently. After baking soda was added, the puree of the radish
darkened from a light pink to a dark violet color. After acetic acid was added to the puree of the radish,
the puree of the vegetables turned into an orange pinkish hue.
Although not all of the vegetables had a huge color change due to the baking soda and the
acetic acid, most had one that was visible to the eye which concluded that baking soda and acetic had an
effect on the color change of vegetables. There were a few vegetables that did in fact stay the same color
as the control after baking soda and acetic, but after being roasted, boiled, steamed, and fried, their pH
values had changed. In conclusion, the effect of heat to vegetables is not only textural change, but it also
changes the pH values of the vegetables. After adding baking soda and acetic acid to the vegetables, there
is a color change.
Recommendations
The only slight problems with the experiment were when the wrong about of baking soda of acetic
acid were added or when pictures weren’t taken. These all resulted in having to start over on the procedure. For
example, the acetic acid and baking soda were all added using pipets. When the wrong amount of baking soda
was added, the vegetables had to be re-blended and re-pureed, so the right amount of vinegar could be added.
When the wrong amount of baking soda was added, which happened many times, the procedure also had to be
redone.
In order to not make this mistake again, it is vital to use the same supplies that the same supplies are
used so the right amount of materials are added. Otherwise, different amounts of the materials are put in and the
results are corrupted. Another mistake that frequently happened was forgetting to take pictures. This experiment
requires pictures to see the color change after each experiment and many pictures were forgotten. In result of
this, the entire procedure had to be redone and the pictures were taken.
To not make this mistake again, it is just very important to remember to take pictures after every
trial. It doesn’t matter if extra pictures are taken, but if some pictures are missing, then the entire procedure has
to be redone. One final error that happened frequently was bumping the results are spilling them onto the paper
beneath it. This also resulted in having to redo the procedure many times.
To avoid this mistake, it is very significant to be careful around the containers containing the
liquids. One small movement around them can cause it to spill and that means redoing the trial because the
results are gone and the white paper used beneath is ruined. Another tip to not spilling the vegetable juices is to
not put too much liquid in the containers. The mistake of filling up the containers all the way to the rim
happened many times which resulted in spilling the juices and having to redo the procedure.
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